This invention relates generally to control systems having one or more redundant control elements to provide continuity of operation in the event that one or more of the elements should fail. More particularly, the invention relates to attitude control systems for space vehicles, such as satellites.
For satellites to perform their intended functions, precise control of their attitude with respect to three orthogonal axes is essential. This is particularly true of communication satellites, which are typically placed in a geosynchronous or twenty-four-hour orbit, so as to remain essentially above the same point on the earth's surface. The attitude of such a satellite must be continuously corrected to maintain one face of the vehicle constantly oriented towards the earth, and also to compensate for any external disturbance torques acting on the vehicle. One well known technique for attitude control of satellites utilizes the principle of the reaction wheel. In simple terms, whenever the speed of a rotatable wheel is changed, an acceleration torque must be applied to it, usually by a drive motor, and an equal and opposite reaction torque is exerted on the motor. If the wheel and drive motor are mounted in a satellite, the reaction torque is applied to the satellite, and tends to accelerate it in the opposite direction to the wheel.
Another way of viewing operation of the reaction wheel is from the standpoint of conservation of momentum. When the angular momentum [speed] of a reaction wheel is, for example, increased, there is a corresponding increase in the angular momentum of the vehicle in the opposite direction, thereby preserving a constant total angular momentum with respect to inertial space. Each reaction wheel is controlled by a relatively high-bandwidth speed control loop, and attitude control is achieved by commanding wheel speeds, and hence angular momenta, as functions of attitude error.
It will be apparent that at least three such reaction wheels are required to effect attitude control in three orthogonal vehicle axes. The three axes are usually defined as the roll, yaw and pitch axes. For an earth-oriented satellite, the roll axis is parallel with the direction of oribital velocity of the satellite, the yaw axis is aligned with the local vertical direction, i.e., with a radial line through the center of the earth, and the pitch axis is orthogonal to both the roll axis and yaw axis.
If three reaction wheels are orthogonally arranged in the vehicle, they can be utilized to control the angular speed and attitude of the vehicle with respect to its three axes. Such a control system, however, would be rendered inoperative upon the failure of any one of the reaction wheels, since the remaining two wheels can have no effect on the vehicle's attitude with respect to the axis corresponding to the failed wheel. For this reason, more than three wheels are typically used.
One well known way of providing failure redundancy is to include a standby redundant wheel for each of the orthogonal wheels. This requires a total of six wheels, two for each control axis, and is commonly known as an "orthogonal redundant" system. Another way of providing failure redundancy is to use four or more wheels oriented in directions that are skewed with respect to the orthogonal vehicle axes. Each skewed-axis wheel contributes torque or momentum to more than one vehicle axis, and any desired set of momentum commands in the three vehicle axes can be given effect by means of a corresponding set of momentum commands applied to the wheels. If one of the wheels should fail, a different set of momentum commands must then be applied to the remaining wheels to obtain the same desired set of momentum commands in the three vehicle axes. So long as at least three reaction wheels having non co-planar axes remain operative, it is always possible to obtain the desired set of momentum commands in the three vehicle axes.
The afore-described use of redundant reaction wheels for the control of space vehicle attitude is well known to designers of such systems. For example, U.S. Pat. No. 4,071,211 entitled "Momentum Biased Active Three-Axis Satellite Attitude Control System", and issued in the name of Muhlfelder et al, discloses a typical example of a four-wheel attitude control system. Such four-wheel systems are often arranged with the wheels in a pyramid configuration, the wheel axes being equally spaced about one of the vehicle axes, such as the pitch axis, and being equally inclined to the plane in which the other two axes lie, such as the roll-yaw plane.
Although the basic technique of employing one or more redundant reaction wheels for satellite attitude control has been known for some years, there has heretofore been no effective technique for rapidly and autonomously detecting the failure of a reaction wheel, and for compensating for the failed wheel in such a manner that the vehicle attitude is still controlled as intended during a failure transient. Accordingly, there is a clear need for a system for detection of and compensation for a control element failure, which is operative rapidly and effectively enough to maintain continuity of operation and performance of the attitude control system, even during the period of detection and correction. The present invention fulfills this need.